I'd be willing to bet that your colleague implemented on-page events.
By default Google Anaytics only measures page views. Bounces are defined as users who only view one page; even if that user spends hours on one page.
Events allow you to track what happens within the page. You can send GA information about items such as:
- User clicking on buttons
- User scrolling the page
- User spending X minutes on the page
- User typing on the page
- User watching a video on the page
- Additional AJAX content gets loaded
All of those events can be marked as "interactive" which causes GA to say "this user did not bounce." They also could be marked as "non-interactive" which does not affect the bounce rate.
If I'm right, you will see data in the "Behavior" -> "Events" -> "Overview" report starting right when your bounce rate went way down.
A 12% bounce rate when events are implemented is quite normal. It is also very normal for some types of sites to have a 75% bounce rate before events:
- Single page applications (SPAs) only have multiple page views if users refresh or click to the site a second time.
- Many sites provide all the information that most users need right in the landing page. That is common for articles and calculators. I feel that it shouldn't be a "bounce" if the user reads the entire article.
Events are the way to let GA know that you are satisfying your users even when they are only viewing one page.
You may want to review the events that are being sent and make sure that each is being marked as "interactive" or "non-interactive" appropriately.